271 EFFECT OF MARE AGE ON OOCYTE MORPHOLOGY AND DEVELOPMENTAL COMPETENCE AFTER INTRACYTOPLASMIC SPERM INJECTION

Abstract

Reduced fertility in aged mares is associated with delayed early embryo development and lower pregnancy rates, potentially related to oocyte developmental competence. Human oocyte morphology has been associated with developmental potential, although comparative evidence is lacking in the mare. Exogenous FSH may be beneficial in obtaining more oocytes; however, effects on oocyte morphology and competence are unknown. Objectives were to determine if zona pellucida thickness (ZPT), ooplasm volume (OV), and perivitelline space volume (PVSV) were related to mare age or FSH treatment and to cleavage, blastocyst, and pregnancy rates after intracytoplasmic sperm injection (ICSI). Cycles with and without eFSH treatment were alternated; eFSH treatments began in diestrus with a cohort of follicles ≥20 mm. Oocytes were collected by transvaginal aspiration from follicles >30 mm from young (4 to 9 years) and old (>20 years) mares at 20 to 24 h after administration of recombinant eLH. Oocytes were cultured for 18 h in TCM-199 at 38.5°C in 6% CO₂ in air. Sperm were injected 40 ± 1 h after eLH, using frozen sperm from a single ejaculate. Presumptive zygotes were incubated in Dulbecco's modified Eagle's medium/F12 + 10% fetal calf serum at 38.5°C in 5% CO₂, 5%O₂, and 90% N₂. Cleavage (≥2 cells) was recorded 48 h after ICSI. Blastocysts considered viable (formation before 9 d and good quality) were transferred nonsurgically into recipients 3 to 7 days after ovulation. Only pregnancies of fetuses with heart beats were included. Morphological parameters of oocytes (old, n = 40; young, n = 37) were obtained from photographic images taken at ICSI and analyzed by computer-assisted measurement using digital calipers (Spot Software, Diagnostic Instruments, Inc., Sterling Heights, MI, USA). Zona pellucida thickness was averaged from 2 measurements 90° to 180° apart. Ooplasm volume was calculated (4/3πr³) from the average of 2 diameters of the ooplasm 90° apart; and PVSV was calculated as the difference of the vitelline membrane volume and that of the volume at the inner volume of the ZP calculated as an oblate spheroid (4/3πa²b) from the average of 2 diameters. Zona pellucida thickness, OV, and PVSV were analyzed using 2-way ANOVA for main effects of age and treatment and 3-way ANOVA by adding cleavage as a factor. Zona pellucida thickness was less (P = 0.007) for old compared with young (least squares mean SEM of 11.4 ± 0.2 and 12.3 ± 0.2 µm, respectively) with no effect on cleavage, blastocyst, or pregnancy rates. Ooplasm volume was not different (P = 0.14) between old and young (309 036 ± 5373 and 320 544 ± 5639 µm³, respectively) and did not affect cleavage, blastocyst, or pregnancy rates. The PVSV was greater (P = 0.001) in old compared with young (157 505 ± 10 853 and 102 161 ± 11 388 µm³, respectively) and may be related to the lower cleavage (P = 0.03), blastocyst (P = 0.02), and pregnancy (P = 0.05) rates. Treatment with FSH had no effect (P > 0.1) on morphology or embryo development. In this study, ZPT and PVSV differed with mare age and could be of predictive value for oocyte developmental competence.